Park goers bathed in the sun at Mission Dolores Park in San Francisco on Tuesday.
Minh Connors/For the S.F. Chronicle
While Southern California is preparing for a reinvigorated heat wave, the unseasonable warmth will start to crack around the Bay Area this weekend, with temperatures cooling by 5 to 15 degrees.
Thursday will likely be the peak of the heat in our region and the last legitimate shot at San Francisco breaking its all-time March temperature record of 87 degrees. For the third consecutive day, temperatures will soar into the mid to upper 90s in the inland valleys of the East Bay and South Bay.
The records at risk Thursday are not close calls. San Jose is forecast at 94, more than 10 degrees above its March record of 82 set in 1988, according to the National Weather Service. San Rafael is expected to hit 92, topping a record from 1964. Livermore, Redwood City and Napa are all on track to break records dating back decades. Friday looks nearly identical to Thursday before the pattern shifts over the weekend.
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The break in the heat comes from what may seem an unlikely source: the heat itself.
After multiple days of extreme surface warming, the Central Valley and desert interior become so overheated that they generate their own surface low pressure zone, or what meteorologists call a thermal trough. It’s a common occurrence in California during the spring and summer — but is somewhat unusual for March.Â
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That deepening trough sharpens the pressure contrast between the lower pressure conditions inland and the higher pressure values over the Pacific, pulling cooler marine air back onshore. And the unusually cool sea surface temperatures off the Bay Area coast will work to enhance the marine layer as it returns.
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By Saturday, most inland valley temperatures will drop by 10 degrees into the 80s. Sunday brings another cooldown, returning much of the region to the 70s inland and 60s along the coast. Winds will increase through the weekend, and morning fog patterns should re-emerge in the city and inland valleys.
The return to near normalcy will not extend beyond the Bay Area, though. Southern California will be dealing with temperatures well above normal through Sunday. Even there, some relief is in sight as the center of high pressure shifts inland and temperatures ease back a few degrees from record values.
Thursday breakdown
San Francisco: Another very hot day in the city, potentially the hottest of this multi-day stretch. A few clouds will pop up along Ocean Beach and the Presidio in the morning but won’t last. Sunshine rules the skies and temperatures should peak in the low 90s in Noe Valley and the Mission District. Downtown San Francisco should reach the upper 80s, while the Sunset District, Outer Richmond and the Presidio stay in the 60s and 70s. It will be clear and warm overnight with lows in the upper 50s to low 60s.
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North Bay: A few low morning clouds in the Marin Headlands will give way to sunshine and another hot day. Temperatures should rise into the mid to upper 80s in San Rafael, Novato, Napa and Sonoma. Santa Rosa and Petaluma should reach the low 90s. It will be clear and mild overnight, with lows in the 50s and 60s in the valleys and some warmer readings above 1,000 feet.
East Bay: Thursday will likely be the hottest day of the week here. Highs should reach the low 90s in Richmond, Oakland, Berkeley and Hayward. Any of these cities could set an all-time monthly record for March. Walnut Creek, Dublin and Livermore will be similarly hot. The overnight will be warm, with lows in the upper 50s to low 60s.
Pacific Coast: A few patches of clouds are possible along the immediate coast in the morning. Sunshine and an offshore wind will push temperatures into record territory again. Half Moon Bay should top out in the mid-80s, likely setting a new monthly temperature record. Pacifica and Daly City will be similarly warm. An evening sea breeze will ease the heat before a clear and mild overnight with lows in the 50s and 60s.
Peninsula: Warming will be uneven Thursday. A light bay breeze will keep South San Francisco and SFO in the 70s for much of the day before both spike into the mid-80s in the late afternoon. Millbrae, San Mateo and Redwood City will heat up faster, topping out in the low 90s. Several locations are at risk of setting daily or monthly temperature records and it will be warm overnight with lows in the upper 50s.
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South Bay: This will be the hottest spot in the region again on Thursday. Highs should easily hit the low 90s in San Jose and Santa Clara, with Gilroy spiking into the mid-90s. Several locations are likely to set daily or monthly temperature records. Look for mild and clear conditions overnight with lows in the 60s.